The invention relates to a new and improved miniature sensor for the precise measurement and digital encoding of both absolute and relative values of angle or position combined in a single instrument. More particularly, the invention relates to precise angle and position determination using an imaging array and frontal illumination to observe, interpret, and calibrate coded markings on the cylindrical edge of a reference wheel or on a planer face of a linear strip.
Digital encoders convert position or motion into a sequence of digital pulses and are used in a wide variety of control and measurement applications. Incremental encoders keep track of accumulated pulse count to provide a direct measure of motion relative to their initial state. Absolute encoders interpret a given reading, converting it directly to position without the need to retain prior pulse history. Encoders are generally designed specifically for either angle or position measurement, and operate using either field or optical sensing techniques.
Field sensing devices include magnetic encoders as well as resolvers, synchros, inductosyns, and linear variable differential transformers. Magnetic encoders sense motion of a polarized magnet or that of small uniformly spaced magnetic dipoles encoded on a reference scale. The other devices have current carrying coils on their rotor or slider as well as the stator, thus requiring electrification on both sides. Memory is generally required to make a magnetic encoder appear to function as an absolute encoder, along with additional electronics for analog to digital conversion and linearity control. Except for the dipole counter, which is non-absolute, the presence of coils or magnets on the moving side limits the opportunity for significant miniaturization.
Optical encoders are inherently digital, processing light rays passing through or reflected from a coded scale and sensed by one or more photodetectors. The most common type of optical rotary encoder (U.S. Pat. No. 4,945,231) consists of a light source, a coded disk pivoted in a precision ball bearing, and an array of discrete photocells. A light emitting diode (LED) shines through the code disc, having alternate annular segments of transparency and opacity. These coded segments interrupt the light beam creating light pulses that are detected by the photocells. To achieve precision, the segments are laid out on an elaborate code wheel in several concentric tracks. A 10 track encoder is capable of producing 210 or 1024 distinct positions, equivalent to an angular resolution of 0.351 degrees (360/1024).
Greater accuracies, of 14 bits and higher, are available but require a larger diameter code wheel to accommodate additional tracks, or alternatively a finer grating resolution and/or alternate process that can significantly impact the cost of manufacturing. Such inherent limitations present significant obstacles for newer applications where size and cost are critical factors.
In view of the above, it would be desirable to achieve miniaturization in an absolute rotary encoder, with resolution equivalent to or better than that of somewhat larger diameter conventional encoders. It would be further desirable to incorporate internal self-calibrating features into the design, thus enabling some relaxation of the normally high tolerances associated with manufacture and assembly of the product, so as to minimize its cost of production. Still further, it would be desirable to provide the ability to perform both angle and linear position computations using the same apparatus without requiring modification to its internal electronics, or operating software.